Add ability to pass command line options when using ember try beta <command>.

[rwjblue]

I haven't dug into this much yet, but I think it should be possible to run:

ember try <scenario-name> test --server

At the moment (as of 0.0.3) this runs ember test (instead of ember test --server) after setting up the scenario.

[rwjblue]

Not a huge issue, just noticed while playing around with this.

[rwjblue]

I took a stab at a spike for this, but I did it on top of the changes in #12 so I want to hold off on submitting a PR until that has landed (to save extra rebasing and such).

In case I forget to come back to it here is my spike rwjblue@428a661...